Refactoring phasor dynamics constituent models

[PhilipFackler]

Description

This PR removes some duplicate code and makes a small improvement to

Proposed changes

• Add ConstituentModel as common base for components and buses.
• Remove SignalNode from evaluator tree (since it doesn't do anything related)

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• [N/A] There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• [N/A] I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

[pelesh]

Perhaps name the new class EvaluatorImpl instead of ConstituentModel? It would be good to have some naming consistency.

[PhilipFackler]

Perhaps name the new class EvaluatorImpl instead of ConstituentModel? It would be good to have some naming consistency.

I changed the name to GridElement as discussed.